Data commit

Task/Animation/Racket/animation-1.rkt

@@ -0,0 +1,40 @@
+#lang racket/gui
+
+;; One of 'left or 'right
+(define direction 'left)
+
+;; Set up the GUI
+(define animation-frame%
+  (class frame%
+    (super-new [label "Animation"])
+    ;; reverse direction on a click
+    (define/override (on-subwindow-event win evt)
+      (when (send evt button-down?)
+        (set! direction
+              (if (eq? direction 'left)
+                  'right
+                  'left))))))
+
+(define frame (new animation-frame%))
+(define msg (new message%
+                 [label "Hello World! "]
+                 [parent frame]))
+
+;; Timer callback to adjust the message
+(define (callback)
+  (define old-label (send msg get-label))
+  (define len (string-length old-label))
+  (if (eq? direction 'left)
+      (send msg set-label
+            (string-append (substring old-label 1)
+                           (substring old-label 0 1)))
+      (send msg set-label
+            (string-append (substring old-label (- len 1) len)
+                           (substring old-label 0 (- len 1))))))
+
+;; Set a timer and go
+(define timer (new timer%
+                   [notify-callback callback]
+                   [interval 500]))
+
+(send frame show #t)

Task/Animation/Racket/animation-2.rkt

@@ -0,0 +1,42 @@
+#lang racket
+(require racket/match)
+(require 2htdp/image)
+(require 2htdp/universe)
+
+; program state
+; value -> no. of rotations of string
+; step -> number (positive for right rotations, else negative)
+(struct rotations (value step) #:transparent)
+
+(define STR "Hello World! ")
+(define LEN (string-length STR))
+
+; font
+(define COLOR "red")
+(define TEXT-SIZE 16)   ;; pixels
+
+(define (render rots)
+  (define val (rotations-value rots))
+  (text (string-append (substring STR val)
+                       (substring STR 0 val))
+        TEXT-SIZE
+        COLOR))
+
+(define (update rots)
+  (match rots
+      [(rotations val dir)
+       (rotations (modulo (+ val dir) LEN) dir)]))
+
+; reverse direction on mouse click
+(define (handle-mouse rots x y event)
+  (case event
+    ; "button-up" indicates mouse click
+    [("button-up") (rotations (rotations-value rots)
+                              (- (rotations-step rots)))]
+    [else rots]))
+
+; start GUI, with given event handlers
+(big-bang (rotations 0 1)
+  [to-draw render]
+  [on-tick update]
+  [on-mouse handle-mouse])